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DEVELOPMENT OF THE UROGENITAL ORGANS. 1141 
urogenital part of the cloaca, which in the course of development becomes transformed into 
the bladder and the urogenital canal of the embryo (Figs. 780 and 781). The developing 
ureter at first opens as a diverticulum from the Wolffian duct, at a short distance from the 
point where the latter joins the cloaca. Soon, however, the ureters acquire independent 
openings into the cloaca, which become gradually shifted further from one another and 
from those of the Wolffian ducts. The ureters are now found to open into the anterior 
portion of the cloaca which lies nearer to the head of the embryo than the part with 
which the Wolffian ducts are connected. This cephalic portion receiving the ureters 
becomes, in the male, the bladder and the upper part of the prostatic urethra; in the 
female, the bladder and the entire urethra a (Fi ig. 782). The caudal part, lying below the 
level of the entrance of the Wolffian ducts, is the urogenital canal, and is represented in 
the adult male by the lower part of the prostatic and the membranous urethra ; in the 
female by the part of the urogenital fissure which immediately surrounds the openings of 
the urethra and vagina (Fig. 782). The united Millerian ducts open into the lower part 
of the cloaca or urogenital canal, between the Wolffian ducts of opposite sides. In the 
male the position of this opening, which is represented in the adult by the opening of the 
uterus masculinus, remains almost unchanged ; in the female, on the other hand, the 
urogenital canal becomes so much shortened during development, that the opening, w ice 
in this sex is represented by the orifice of the vagina, comes to the surface at the bottom 
of the urogenital fissure (Fig. 782). The cavity ‘of the developing bladder is directly con- 
tinuous above with that of the allantois, and by some authorities the bladder is described 
as arising as a dilatation of the proximal part of the allantois. 
When the ectodermal cloacal fossa is formed, after the complete separation of the 
cloaca into anterior or urogenital, and posterior or rectal ae it communicates with the 
rectum behind and with the urogenital canal in front (p. 43). The fossa is bounded 
laterally by a well-narked skin fold, and at its anterior end is situated the sexual eminence. 
This eminence becomes the ter aerial portion of the penis or of the clitoris, and its posterior 
surface is grooved longitudinally in such a manner that the proximal end of the groove 
lies near the opening of the urogenital canal into the cloacal fossa. 
In the male the lips of this groove on the sexual eminence, meeting together, form a 
canal which, in the adult, traverses the glans penis as the terminal part of the urethra. 
The lateral margins of the cloacal fossa, behind the sexual eminence, meet together, and 
fusing in the middle line, convert the fossa into a canal, which becomes the spongy portion 
of the adult urethra. In this way all the cloacal fossa lying in front of the rectum 
becomes closed over to form the third part of the urethra. The last portion of the fossa 
to disappear from the surface is the anterior part, which lies just at the base of the sexual 
eminence (Fig. 782). The spongy part of the urethra is thus derived from the ectodermal 
cloacal fossa, while the prostatic and membranous parts owe their origin to the endodermal 
cloaca. 
In the female the margins of the cloacal fossa remain separate throughout life, and the 
fossa persists as the urogenital space. The groove on the sexual eminence, or glans 
clitoridis, closes over, but does not form a canal as in the male. The urogenital canal, at 
first like that of the male, becomes shorter and shorter during development. | Owing to 
this shortening the lower end of the fused portions of the Miillerian ducts, which opens 
into the canal, and from which the vagina is formed, appears in the adult to open directly 
into the fossa (urogenital space) behind the channel leading from the bladder. From the 
latter channel is formed the female urethra (Fig. 782). 
Wolffian Duct and Embryonic Secretory Organ.—The Wolffian duct 
arises in the mesoblast, about the fifteenth day, as a solid cord of cells occupying 
a position immediately to the outer side of the mesoblastic somites and to the inner 
side of the body cavity. When first recognised the duct les immediately beneath 
the epiblast, and as it grows backwards to reach the cloaca it is often found to be 
intimately connected with the epiblast. This close connexion of the duct with the 
epiblast, in the early stages, is by some authorities supposed to indicate a primitive ecto- 
dermal origin of the canal, but by others, and apparently with more reason, to be a trace 
of the opening of the ducts on the surface of the body, such as exists in connexion with 
the excretory organs of lower animals. During the third week the cellular cord which 
represents the Wolffian duct acquires a lumen, and about the end of the same week the 
duct in its growth reaches the cloaca. As soon as the cloaca has become divided into 
dorsal and ventral subdivisions, the Wolffian duct is seen to end in the caudal part of the 
ventral subdivision, which becomes the urogenital canal (Fig. 781). 
The Wolffian body or mesonephros is dev eloped in the mesoblast of the “intermediate 
cell mass,” immediately to the outer side of the Wolffian duct, and consists of a number 
